The present invention relates to a system and method of controlling a variable steering ratio of a vehicle steer-by-wire system by applying fuzzy logic technology.
The vehicle steering ratio is one of the critical specifications for a vehicle steering system. Typically, the steering ratio of a vehicle represents a proportional factor between a steering wheel angle and a road wheel angle. For example, if a vehicle steering ratio is 16, the road wheel angle will turn 1 degree when the steering wheel is turned 16 degrees. Most existing vehicles with a conventional steering system have a fixed steering ratio, wherein that the steering ratio remains substantially constant except for minor variations due to the suspension geometry of the vehicle.
In order to improve the steering performance and to reduce the skill level requirement of a vehicle driver, some automotive manufacturers have implemented a variable ratio rack and pinion gear steering system. With a variable ratio steering gear set, the range and adjustment of the variable ratio are limited by the mechanical structure of the gears. This type of gear set only provides a variable ratio as a function of steering wheel angle. Specific manufacturing technologies are needed to produce a variable ratio rack and pinion gear increasing the product cost and investment requirements.
In a steering system that does not rely on a mechanical connection between the steering wheel and the road wheels, it is possible to vary the steering ratio with the steering wheel angle, the vehicle speed and other vehicle variables that can improve the steering performance using the variable steering ratio. The steer-by-wire system can generate a variable steering ratio. The variable ratio feature can be implemented using software in a steer-by-wire system, such that the product cost, development time and investment are significantly reduced compared to conventional steering system solutions.
It has been known that desired vehicle steering performance results from higher steering ratios on center for improved stability and lower ratios off center, reducing required lock-to-lock turns for easier parking. Furthermore, it is desirable to reduce the sensitivity of the steering ratio with the increase of vehicle speed. It is also possible for the steering system to provide adjustable variable steering ratios in the same vehicle, which could be set by the vehicle driver or automatically based on driving behavior trends. Therefore, it is a challenge for steer-by-wire systems to provide a flexible application environment to implement the variable steering ratio control and achieve the variable ratio steering functions mentioned above.
The present invention generally provides a system and method of controlling a variable steering ratio of a vehicle steer-by-wire system by applying fuzzy logic technology. The vehicle steer-by-wire system can be considered as an integrated control system which is comprised of two different parts according to their functions: a steering wheel control system and a road wheel control system. The steering wheel control system provides the steering feel for the vehicle driver and the steering wheel angle command signal. The road wheel control system provides the actual road wheel angle tracking to the road wheel reference angle signal. The steer-by-wire system further includes a variable steering ratio regulation unit to receive a steering wheel angle signal from the steering wheel system and a vehicle speed signal from the vehicle. The variable steering ratio regulation unit generates the road wheel reference angle to the road wheel system. The variable steering ratio regulation unit provides a different road wheel reference angle based on changes in steering wheel angle and vehicle speed by using fuzzy logic control technology. As a result, the vehicle""s steering and handling performance is improved through a broad range of driving conditions.
In one embodiment, the variable steering ratio regulation unit is in electrical communication with both the steering wheel control system and the road wheel control system. The steering wheel control system and the road wheel control system are in electrical communication with each other, wherein the variable steering ratio regulation unit is electrically linked therebetween. This allows the steering ratio regulation unit to receive a steering wheel angle signal from the steering wheel system and signals from vehicle, and generate a steering directional road wheel reference angle to the road wheel system. This also allows the steering wheel system to control the steering wheel reaction torque to produce a realistic steering feel to a driver of the vehicle. This further allows the road wheel system to track a road wheel reference angle based on the steering wheel input from the driver and to maintain alignment between the steering wheel and the road wheels of the vehicle.
The variable steering ratio regulation unit applies fuzzy logic technology to implement the variable steering ratio control, and it further provides a variable steering ratio selection using fuzzy logic inference. The fuzzy logic technology may incorporate imprecise multiple input information including the steering wheel angle and vehicle speed to obtain an optimal smooth solution for variable steering ratio. This invention describes the fuzzy logic based steering ratio control method implemented in the variable steering ratio regulation unit. The fuzzy logic based steering ratio control operates in three steps: fuzzification, inference and defuzzification. All crisp input and output variables including steering wheel angle, vehicle speed and road wheel reference angle are converted into values in the fuzzy sets by defining labels and membership functions. Then, using labels and membership functions as defined in the stage of fuzzification, a set of rules for the fuzzy inference stage are given to describe an optimal road wheel reference angle output based on system requirements and development experience. The road wheel reference angle in a linguistic value description is finally converted to a crisp value in the stage of defuzzification.
The present invention also describes a new development of variable steering ratio control with an adjustable variable steering ratio function. The steering ratio will not only vary with the steering wheel angle and vehicle speed, but it will also vary according to the type of driving and environmental situation. The adjustable variable ratio function could be set by the vehicle operator or it could be set automatically based on the type of driving and environmental situations. The vehicle variables that cause the steering ratio adjustment can be detected using sensors and estimated using other variables.
Further aspects, features, and advantages of the invention will become apparent from consideration from the following description and the appended claims when taken in connection with the accompanying drawings.